This invention relates to adhesives and particularly to subcoats for bonding synthetic fibers, such as aramid, nylon and polyester fibers, to rubber. Such compositions may be used in manufacturing composite articles, for example tires, to bond reinforcement fibers to tire rubber compositions.
In the manufacturing of rubber composites which include fiber reinforcement in the form of fibers, filaments or cords, close attention is given to the adhesion between the rubber matrix of the composite and the fibrous reinforcement.
Aramid fiber, for example, is a fiber used for tire reinforcement. Providing adhesion between aramid and rubber is very difficult due to the crystalline surface of the fiber. Hence a separate subcoat is necessary in order to form a reasonable bond between fiber and rubber. An epoxy adhesive subcoat followed with a resorcinol formaldehyde latex (RFL) dip provides good adhesion to sulfur cured rubber for aramid reinforcement.
U.S. Pat. No. 5,922,797, incorporated herein by reference, teaches the use of a latex adhesive for improving the adhesion between a rubber matrix and fibrous reinforcement.
U.S. Pat. No. 2,748,049 discloses an adhesive dip which is reported to strongly adhere to fabric and rubber, and which is further reported to be relatively flexible. This dip is made utilizing a latex of a copolymer of a conjugated diolefin monomer, such as butadiene, and an unsaturated aldehyde monomer, such as methacrolein.
In commercial practice, resorcinol-formaldehyde-latex (RFL) dips have been widely implemented by the tire industry for bonding synthetic fabrics to rubber. For instance, U.S. Pat. No. 3,525,703 discloses a water-based adhesive composition for bonding synthetic fiber material to rubber. The teachings of U.S. Pat. No. 3,525,703 specifically disclose the utilization of styrene-butadiene latex and vinylpyridine-styrene-butadiene latex in such water-based adhesive compositions.
U.S. Pat. No. 4,460,029, incorporated herein by reference, teaches the use of oxazoline polymerized in latex form for tire cord adhesion.
Nippon Shokubai Co., Ltd., has acquired U.S. Pat. No. 4,460,029, and has continued to develop oxazoline compositions. Product information on oxazolines has been published by Nippon Shokubai Co., Ltd., entitled xe2x80x9cEPOCROS K-1000, K-2000 seriesxe2x80x9d, whereby such publication is also incorporated herein by reference.
Epoxy subcoat used currently demonstrates excellent adhesion with good rubber coverage in all compounds.
Due to environmental concerns regarding epoxy material, there is an interest in finding an alternative subcoat material, if substantially equivalent adhesion is maintained.
Also, oxazoline polymer is known to provide much longer pot life in adhesive formulations than epoxies.
A composition comprising 1% to 4% solids oxazoline polymer, 0% to 4% alkylene glycol monoether, and 92% to 99% water is provided, wherein all percentages are by weight.
In an alternative embodiment, the composition may comprise (1) oxazoline polymer and (2) a latex which is comprised of (a) water, (b) an emulsifier and (c) a polymer.
Also provided is a process for adhering synthetic textile fibers to sulfur vulcanizable rubbers comprising the steps of (I) immersing said fibers in an aqueous dispersion which is comprised of (a) 1% to 4% by weight oxazoline polymer, (b) 0% to 4% alkylene glycol monoether and (c) 92% to 99% water to produce coated fibers; (II) drying the coated fibers to produce subcoated fibers; (III) subjecting the subcoated fibers to an RFL adhesive dip to produce dipped fibers; (IV) drying the dipped fibers to produce dried dipped fibers; (V) placing the dried dipped fibers in contact with a sulfur vulcanizable rubber; and (VI) curing the sulfur vulcanizable rubber while it is in contact with the dried dipped fibers.
Also provided is a composite article of rubber and fibers, wherein the fibers have distributed over surface portions thereof (a) oxazoline polymer; (b) an RFL adhesive; and (c) an in situ vulcanized rubber compound.
The fibers in the composite article may comprise, for example, polyester fibers and aramid fibers.
In a preferred embodiment, the composite article is a tire.
Schuetz et al., in U.S. Pat. No. 4,460,029 teach the use of oxazoline in a water insoluble latex adhesive for improving adhesion between fiber reinforcement (especially filaments and cords) and rubber. According to the present invention, the inventors have modified a water soluble oxazoline polymer solution (without latex) provided by Nippon Shokubai by diluting the solution with water to provide a solution with a polymer solids content of 1% to 4%, and have used the diluted solution directly on substrate fibers as a subcoat (in place of an epoxy subcoat), prior to a resorcinol aldehyde latex (RFL) dip, to improve adhesion between the fibers and a rubber matrix in a composite in which the fibers may be used.
In one embodiment, the oxazoline polymer solution contains an alkylene glycol monoether solvent (WS-500). For environmental reasons, it is preferred that the oxazoline polymer be prepared without solvent, although compositions made with or without glycol monoether solvent show desirable results in the invention.
The diluted oxazoline polymer, when used as a subcoat on fibers, provides good adhesion to the types of rubber compositions described in U.S. Pat. No. 4,460,029.
Oxazoline polymers, EPOCROS WS-500 and WS-700, developed by Nippon Shokubai Co., Ltd., are water-soluble polymers containing pendant oxazoline groups which are designed for use as a crosslinker for carboxylated waterborne polymers.
Oxazoline reacts with functional groups such as carboxyl, acid anhydride, phenolic hydroxyl and phenolic thiol. The reaction of an oxazoline group with a carboxylic group is very effective. It can react rapidly with carboxylic groups above temperatures of 80 to 100 degrees C to form an amide ester. 
Because of its high reactivity, the inventors speculated that the oxazoline crosslinker may have potential for use in an adhesive system for rubber reinforcement materials.
Early tests of the material provided by Nippon Shokubai did not show good results, and showed different degrees of effectiveness on different materials. Since some tests showed an over stiffening of the fiber, the inventors further speculated that a reduction of the solids content of adhesive would improve the results.
Oxazoline polymer WS-500 has a composition (% by wt) of polymer/PM/water 40/38/22, where PM is propylene glycol monomethylether. Although the inventors dilute the polymer to much lower solid level for subcoat applications, the presence of organic solvent could still affect the environment. Based on this concern, the inventors suggested, and Nippon Shokubai made an effort to produce a new oxazoline polymer (WS-700) which is free of volatile organic chemicals (VOC). WS-700 has a solid level of 24.1%. The weight per oxazoline equivalent (WPO) of WS-700 is 220 (gram solid /equivalent), the same as in WS-500.
In accordance with this invention, the cord or fabric to be treated is dipped for 1 to 5 seconds in the oxazoline polymer solution of the invention, followed by dipping in an RFL latex adhesive, and dried at a temperature within the range of about 75xc2x0 C. to about 265xc2x0 C. for about 0.5 minutes to about 5 minutes, and thereafter calendered into rubber and cured therewith. The drying step utilized will preferably be carried out by passing the cord through 2 or more drying ovens which are maintained at progressively higher temperatures. For instance, it is highly preferred to dry the cord by passing it through a first drying oven which is maintained at a temperature of about 250xc2x0 F. (121xc2x0 C.) to about 300xc2x0 F. (149xc2x0 C.) and then to pass it through a second oven which is maintained at a temperature which is within the range of about 350xc2x0 F. (177xc2x0 C.) to about 500xc2x0 F. (260xc2x0 C.). It should be appreciated that these temperatures are oven temperatures rather than the temperature of the cord being dried. The cord will preferably have a total residence time in the drying ovens which is within the range of about 1 minute to about 3 minutes. For example, a residence time of 30 seconds to 90 seconds in the first oven, and 30 seconds to 90 seconds in the second oven, could be employed.
In testing, the oxazoline subcoat of the invention demonstrated adhesion comparable to the control when using an oxazoline solution having a solid content of 1% to 4% by weight. Some of the fibers tested appeared to be too stiff when the 4% solution was used, and slightly reduced adhesion was observed in some fibers when the 1% solution was used. The best overall results were achieved using a 2% oxazoline solution on a substrate, but it is believed that higher and lower concentrations will be useful in specific applications where stiffness is not a factor or in components where reduced adhesion is acceptable.
It was found that the diluted composition of the invention can be used in combination with a latex to show good adhesion, and such an oxazoline polymer/latex combination can especially be used with polyester cords to reduce stiffness.
As is known to those skilled in the art, latexes are made with an emulsifier, a polymer and water. It is believed that latex compositions described in the references herein can be used with the oxazoline polymer composition when used as a subcoat. Such latexes may be used in a solution having a solids content of 0.5% to 4%, preferably about 1% latex, together with 0.5% to 4% oxazoline polymer, where the total solids content is 1% to 6%, and all percentages are by weight. Preferred are latexes containing 1,3-butadiene, copolymers or terepolymers thereof with styrene and vinyl pyridine as the polymer component.
As used herein, xe2x80x9cfibersxe2x80x9d includes short fibers, as well as fiber cords and filaments. The fibers may comprise organic polymers known in the art, such as, but not limited to, polyamides, aromatic polyamides, polyesters, polyalchohols, polyalkylenes and polyethers, as well as inorganic fibers such as fiber glass and carbon fibers (e.g. graphite).